New approach for correction of error associated with keratometric estimation of corneal power in keratoconus

Accuracy of intraocular lens power formulas in eyes with keratoconus: Multi-center study in Japan

PURPOSE

To assess the accuracy of intraocular lens (IOL) power formulas, namely, SRK/T, Haigis, Barrett Universal II, Barrett True-K for keratoconus, Kane formula, and Kane formula for keratoconus, for cataract with keratoconus in Japanese eyes.

SETTING

Five surgical sites in Japan.

DESIGN

A retrospective case series.

METHODS

Eyes with keratoconus undergoing cataract surgery were included. Postoperative refraction was compared with the prediction by the formulas. Visual acuity, manifest spherical equivalent, prediction error (PE), and mean absolute errors (MAEs) were determined 1 month postoperatively. The PE within 0.50 diopter (D), 1.00 D, and 2.00 D were compared between IOL formulas. Subgroup analysis based on the steepest keratometry (stage 1, ≤ 48 D; stage 2, > 48 D and ≤ 53 D; and stage 3, > 53 D) was performed. The relationship between PE and preoperative biometric data were assessed.

RESULTS

Fifty eyes were included. The MAE of the Barrett True-K for keratoconus, Kane keratoconus, and Kane formulas were significantly lower than that of Haigis. A statistically significant difference in the prediction accuracy within ± 0.50 D was found between Kane keratoconus and Haigis. The prediction accuracy of the Barrett True-K for keratoconus, SRK/T, and Kane within ± 1.00 D was statistically significant compared with that of Haigis. In stage 3, the Barrett True-K for keratoconus had a significantly lower MAE than SRK/T and Haigis.

CONCLUSION

Keratoconus-specific formulas were more accurate than existing formulas in Japanese eyes. The Barrett True-K formula for keratoconus had higher prediction accuracy in severe keratoconus.

Differences Between Simulated Keratometry and Total Corneal Power in Eyes With Keratoconus and a Formula to Improve IOL Power Calculation Results

PURPOSE

To compare simulated keratometry (SimK) and total corneal power (TCP) in keratoconic eyes, to determine whether the differences are systematic and predictable and to evaluate an adjusted TCP-based formula for intraocular lens (IOL) power calculation.

METHODS

In a consecutive series of keratoconic eyes, measurements of SimK, TCP, posterior keratometry, and anterior and posterior corneal asphericities (Q-values) were retrospectively collected. The difference between SimK and TCP was linearly correlated to the biometric parameters. In a separate sample of keratoconic eyes that had undergone cataract surgery, IOL power was calculated with the Barrett Universal II, Hoffer QST, Holladay 1, Kane, and SRK/T formulas using the SimK and an adjusted TCP power. The respective prediction errors were calculated.

RESULTS

A total of 382 keratoconic eyes (271 patients) were enrolled. An increasing overestimation of SimK by TCP was detected from stage I to III, with a significant correlation between the SimK and TCP difference and SimK in the whole sample (P < .0001, r2 = 0.1322). Approximately 7% of cases presented an underestimation of SimK by TCP. IOL power calculation with the adjusted TCP improved outcomes, achieving a maximum of 80% of eyes with a prediction error within ±0.50 diopters with the Hoffer QST, Holladay 1, and Kane formulas.

CONCLUSIONS

Overall, SimK overestimated TCP. Such a difference could not be predicted by any variable. The proposed TCP-adjustment formula (TCPadj = TCP + 0.56 diopters) in keratoconic eyes for IOL power calculation might be valuable for improving refractive outcomes. [J Refract Surg. 2024;40(4):e253-e259.].

Intraocular Lens Power Calculations in Keratoconus Eyes Comparing Keratometry, Total Keratometry, and Newer Formulae

PURPOSE

To compare the utility of keratometry vs total keratometry (TK) for intraocular lens power calculations in eyes with keratoconus (KCN) using KCN and non-KCN formulae.

DESIGN

Retrospective cohort study.

METHODS

This study was conducted at 2 academic centers and included 87 eyes in 67 patients who underwent cataract surgery between 2019 and 2021. Biometry measurements were obtained using a swept-source optical coherence tomography biometer (IOL Master 700). Refractive prediction errors, including root mean square error (RMSE), were calculated for 13 formulae. These included 4 classical formulae (Haigis, Hoffer Q, Holladay 1 [H1], and SRK/T), 5 new formulae (NF) (Barrett Universal II [BU2], Cooke K6, EVO 2.0, Kane, and Pearl-DGS), 3 KCN formulae (BU2 KCN: M-PCA, BU2 KCN: P-PCA, and Kane KCN), and H1 with equivalent keratometry reading values (H1-EKR). Formulae were ranked by RMSE. Friedman analysis of variance with post hoc analysis and H-testing was used for statistical significance testing.

RESULTS

KCN formulae had the lowest RMSEs in all eyes, and BU2 KCN:M-PCA performed the best among KCN formulae in all subgroups. In eyes with severe KCN, if TK values are unavailable, the BU2 KCN: P-PCA performed better than the top-ranked non-KCN formula (SRK/T). In eyes with nonsevere KCN, if TK values are unavailable, EVO 2.0 K was statistically superior to the next competitor (Kane K). H1-EKR had the highest RMSE.

CONCLUSIONS

KCN formulae and TK are useful for intraocular lens power calculations in KCN eyes, especially in eyes with severe KCN. The BU2 KCN: M-PCA using TK values performed best for eyes with all severities of KCN. For eyes with nonsevere KCN, the EVO 2.0 TK or K can also be used.

Cataract surgery in patients with underlying keratoconus: focused review

An underlying diagnosis of keratoconus (KC) can complicate cataract surgery. In this study, the results of a focused review of the literature pertaining to cataract surgery in patients with KC are detailed. Topics essential for the appropriate management of this patient population are discussed. First, the individual and shared epidemiology and pathophysiology of cataract and KC are reviewed. Then, the theory and approach to intraocular lens power calculation are discussed, highlighting particularities and pitfalls of this exercise when performed in patients with KC. Finally, several special-although not uncommon-management scenarios and questions are addressed, such as surgical planning in cases where corneal stabilization or tissue replacement interventions are also necessitated.

Comparison of Four Intraocular Power Calculation Formulas in Keratoconus Eyes

Introduction:

This study aimed to evaluate the differences in Intraocular Lens (IOL) power in keratoconus (KC) eyes between calculations obtained clinically with the most commonly used formulas in healthy eyes (SRK T, Holladay 1, Hoffer Q and Haigis) as well as to define predictive factors for such differences.

Methods:

This retrospective study comprised 43 keratoconus eyes of 22 patients with no previous ocular surgery. IOL powers were calculated with SRK T, Holladay 1, Hoffer Q, and Haigis formulas, considering the Effective Lens Position (ELP) of each formula and the desired refraction of 0 D (Rdes=0 D).

Results:

All differences between formulas were statistically significant and clinically relevant. Haigis formula always provided higher values compared to the rest of the formulas, with the highest differences observed when comparing Haigis with Hoffer (0.84 D) and Hoffer Q (1.17 D) formulas. The lowest difference was obtained for the comparison between SRK-T and Holladay 1 formulas (0.22 D). Differences of the Haigis formula compared to the rest were higher as the magnitude of the IOL power calculated decreased, becoming the patient more myopic. Increased differences between Haigis and Hoffer formulas were observed in eyes with deep anterior chambers, steeper anterior and posterior corneal surfaces, and high axial lengths.

Conclusion:

The most comparable results in IOL power in keratoconus are provided by the Holladay 1 and SRK T formulas, whereas the Haigis formula provides the most discrepant outcome. The consideration of the curvature of the second corneal surface in IOL power calculations in keratoconus may decrease the variability between calculation methods. However, other factors as anterior chamber depth or axial length are also relevant.

Toric intraocular lens power calculation in cataract patients with keratoconus

Purpose

Intra-ocular lens (IOL) power calculation in eyes with keratoconus typically results in hyperopic postoperative refractive error. We investigated the visual and refractive outcomes in keratoconus patients having cataract surgery with a toric IOL and compared IOL power calculation accuracy of conventional formulae and keratoconus specific formulae.

Setting

Ein-Tal Eye Center, Tel-Aviv, Israel.

Design

Retrospective case-series study.

Methods

Post-operative visual acuity and manifest refraction were examined. The error in predicted refraction and IOL power calculation accuracy within a range of 0.5 to 2.0 diopters were compared between different IOL calculating formulae.

Results

Thirty-two eyes with keratoconus were included. Visual acuity improved in all cases and subjective astigmatism decreased from -2.95+/-2.10 D to -0.95+/-0.80 D (p<0.001). Mean absolute errors were: Barrett True-K for keratoconus with measured or predicted posterior corneal power, 0.34 D; Barrett Universal II, 0.64 D; Kane Formula, 0.69 D; Kane Formula for keratoconus, 0.49 D; SRK/T, 0.56 D; Haigis, 0.72 D; Holladay 1, 0.71 D and Hoffer Q, 0.87 D. Barrett True-K formula with measured posterior corneal power, SRK/T and Kane Formula for keratoconus resulted in a prediction error within 0.5 D of 87.5%, 59.4% and 53.1%, respectively.

Conclusions

Cataract removal with a toric IOL significantly improves visual acuity and decreases astigmatism in keratoconic eyes with a topographic central relatively regular astigmatic component. Keratoconus specific formulae resulted in lower mean error in predicted refraction compared with conventional calculating formulae. Utilizing the posterior corneal power within the Barrett True K formula for keratoconus improved IOL power prediction accuracy.

Analysis of total corneal astigmatism with a rotating Scheimpflug camera in keratoconus

Background

To analyze mean corneal powers and astigmatisms on anterior, posterior, and total cornea in patients with keratoconus as calculated according to various keratometric measurements using a Scheimpflug camera.

Methods

We examined the left eyes of 64 patients (41 males and 23 females; mean age 29.94 ± 6.63 years) with keratoconus. We measured simulated K (Sim-K), posterior K, true net power (TNP) and four types of total corneal refractive powers (TCRP). We then used the obtained values to analyze mean K, and corneal astigmatism. TCRP were measured at 2.0 ~ 5.0 mm.

Results

Mean corneal powers from Sim K, posterior K, and TNP were 49.12 ± 3.99, − 7.39 ± 0.79, and 47.78 ± 4.09 diopters, respectively. For TCRP centered on the pupil, mean K tended to decrease with measurement area (all p < 0.01). While, both mean K and astigmatism measured using TCRP centered on the apex decreased with measurement area (all p < 0.001). TCRP centered on the apex were greater than those centered on the pupil for mean K values calculated using TCRP (all p < 0.001). The proportion of WTR was greatest on the anterior and total cornea. As the measurement area moved to the periphery, the proportion of WTR increased.

Conclusions

Mean corneal powers and astigmatisms on total cornea with keratoconus change depending on calculation methods and measurement areas.

Accuracy of Intraocular Lens Power Formulas Modified for Patients with Keratoconus

PURPOSE

To assess the accuracy of intraocular lens (IOL) power formulas modified specifically for patients with keratoconus (Holladay 2 with keratoconus adjustment and Kane keratoconus formula) compared with normal IOL power formulas (Barrett Universal 2, Haigis, Hoffer Q, Holladay 1, Holladay 2, Kane, and SRK/T).

DESIGN

Retrospective consecutive case series.

PARTICIPANTS

A total of 147 eyes of 147 patients with keratoconus.

METHODS

Data from patients with keratoconus who had preoperative IOLMaster biometry were included. A single eye per qualifying patient was randomly selected. The predicted refraction was calculated for each of the formulas and compared with the actual refractive outcome to give the prediction error. Subgroup analysis based on the steepest corneal power measured by biometry (stage 1: ≤48 diopters [D], stage 2: >48 D and ≤53 D, and stage 3: >53 D) was performed.

MAIN OUTCOME MEASURE

Prediction error.

RESULTS

On the basis of the mean absolute prediction error (MAE), the formulas were ranked as follows: Kane keratoconus formula (0.81 D), SRK/T (1.00 D), Barrett Universal 2 (1.03 D), unmodified Kane (1.05 D), Holladay 1 (1.18 D), unmodified Holladay 2 (1.19 D), Haigis (1.22 D), Hoffer Q (1.30 D), and Holladay 2 with keratoconus adjustment (1.32 D). The Kane keratoconus formula had a statistically significant lower MAE compared with all formulas (P < 0.01). In stage 3 keratoconus, all nonmodified formulas had a hyperopic mean prediction error ranging from 1.72 to 3.02 D.

CONCLUSIONS

The Kane keratoconus formula was the most accurate formula in this series. The SRK/T was the most accurate of the traditional IOL formulas. All normal IOL formulas resulted in hyperopic refractive outcomes that worsened as the corneal power increased. Suggestions for target refractive aims in each stage of keratoconus are given.

Current Trends in Modern Visual Intraocular Lens Enhancement Surgery in Stable Keratoconus: A Synopsis of Do's, Don'ts and Pitfalls

Keratoconus is a relatively common ectatic, non-inflammatory corneal disorder that involves gradual visual deterioration through progressive alteration of the shape of the cornea. The corneal thinning, irregular astigmatism and higher order aberrations that occur as the disease progresses pose major challenges in the visual rehabilitation of such patients. This paper summarizes the current literature regarding the results of visual enhancement procedures in patients with stable keratoconus treated with standalone anterior or posterior chamber phakic intraocular lens implantation and monofocal, toric or multifocal toric intraocular lens implantation following phacoemulsification for age-related cataract extraction or refractive lens exchange.

Comparison of Simulated Keratometry and Total Refractive Power for Keratoconus According to the Stage of Amsler-Krumeich Classification

This study was aimed to assess the simulated keratometry (Sim K) and the total corneal refractive power (TCRP) in eyes with keratoconus with respect to the Amsler-Krumeich classification. We enrolled 100 eyes of 100 keratoconic patients and 25 age-matched normal eyes. The Sim K and TCRP were measured with a rotating Scheimpflug system (Pentacam HR, Oculus). The differences between Sim K and TCRP in the keratoconus group were significantly larger than those in the control group (p < 0.001). The differences between Sim K and TCRP became larger in the progressive stages of the disease (p = 0.191 for stage 1, p = 0.008 for stage 2, p < 0.001 for stage 3, p < 0.001 for stage 4). We found a significant correlation of Sim K with the differences between Sim K and TCRP in keratoconic patients (r = 0.497, p < 0.001). The differences between Sim K and TCRP for keratoconus were significantly larger than those for normal eyes, and the differences between Sim K and TCRP tended to become larger in the progressive stages of the disease. It is suggested that the Sim K readings overestimate the TCRP, especially in advanced keratoconus, and that this discrepancy is a possible source of a hyperopic refractive error after cataract surgery.

Time Course of Changes in Simulated Keratometry and Total Corneal Refractive Power after Corneal Collagen Cross-Linking for Progressive Keratoconus

Purpose

To assess the simulated keratometry (Sim K) and the total corneal refractive power (TCRP) in eyes undergoing conventional corneal cross-linking (CXL).

Methods

This study comprised 20 eyes of 20 keratoconic patients (14 men and 6 women; median age (25th and 75th percentile), 26.5 (21.8, 38.0) years) who underwent CXL. The Sim K and TCRP were measured with a rotating Scheimpflug system (Pentacam HR, Oculus), preoperatively and 1, 3, 6, and 12 months postoperatively.

Results

The values of Sim K were 52.65 (46.00, 55.70), 52.45 (45.85, 56.88), 51.70 (45.78, 55.83), 51.40 (45.68, 56.80), and 51.25 (46.08, 56.15) D preoperatively and 1, 3, 6, and 12 months postoperatively, respectively. The corresponding figures of TCRP were 52.10 (45.48, 55.08), 51.30 (45.18, 55.20), 50.95 (45.15, 54.50), 50.00 (45.18, 55.08), and 49.80 (45.48, 54.15) D, respectively. The variances of the Sim K and TCRP data were not statistically significant (p=0.994 and p=0.970, respectively, Kruskal-Wallis test). The Sim K was significantly larger than the TCRP before CXL and at 1, 3, 6, and 12 months after CXL (p<0.001, Wilcoxon signed-rank test).

Conclusions

Not only the Sim K but also TCRP was decreased by approximately 1 D after CXL. The Sim K readings may overestimate the TCRP, even after CXL for progressive keratoconus.

Surgical Options for the Refractive Correction of Keratoconus: Myth or Reality

Keratoconus provides a decrease of quality of life to the patients who suffer from it. The treatment used as well as the method to correct the refractive error of these patients may influence on the impact of the disease on their quality of life. The purpose of this review is to describe the evidence about the conservative surgical treatment for keratoconus aiming to therapeutic and refractive effect. The visual rehabilitation for keratoconic corneas requires addressing three concerns: halting the ectatic process, improving corneal shape, and minimizing the residual refractive error. Cross-linking can halt the disease progression, intrastromal corneal ring segments can improve the corneal shape and hence the visual quality and reduce the refractive error, PRK can correct mild-moderate refractive error, and intraocular lenses can correct from low to high refractive error associated with keratoconus. Any of these surgical options can be performed alone or combined with the other techniques depending on what the case requires. Although it could be considered that the surgical option for the refracto-therapeutic treatment of the keratoconus is a reality, controlled, randomized studies with larger cohorts and longer follow-up periods are needed to determine which refractive procedure and/or sequence are most suitable for each case.

Algorithm for Correcting the Keratometric Error in the Estimation of the Corneal Power in Keratoconus Eyes after Accelerated Corneal Collagen Crosslinking

Purpose

To analyze the errors associated to corneal power calculation using the keratometric approach in keratoconus eyes after accelerated corneal collagen crosslinking (CXL) surgery and to obtain a model for the estimation of an adjusted corneal refractive index (n_kadj) minimizing such errors.

Methods

Potential differences (ΔP_c) among keratometric (P_k) and Gaussian corneal power (P_c^Gauss) were simulated. Three algorithms based on the use of n_kadj for the estimation of an adjusted keratometric corneal power (P_kadj) were developed. The agreement between P_k(1.3375) (keratometric power using the keratometric index of 1.3375), P_c^Gauss, and P_kadj was evaluated. The validity of the algorithm developed was investigated in 21 keratoconus eyes undergoing accelerated CXL.

Results

P_k(1.3375) overestimated corneal power between 0.3 and 3.2 D in theoretical simulations and between 0.8 and 2.9 D in the clinical study (ΔP_c). Three linear equations were defined for n_kadj to be used for different ranges of r_1c. In the clinical study, differences between P_kadj and P_c^Gauss did not exceed ±0.8 D n_k = 1.3375. No statistically significant differences were found between P_kadj and P_c^Gauss (p > 0.05) and P_k(1.3375) and P_kadj (p < 0.001).

Conclusions

The use of the keratometric approach in keratoconus eyes after accelerated CXL can lead to significant clinical errors. These errors can be minimized with an adjusted keratometric approach.

Preliminary validation of an optimized algorithm for intraocular lens power calculation in keratoconus

Purpose:

This study aimed to evaluate the theoretical influence on intraocular lens power (P_IOL) calculation of the use of keratometric approach for corneal power (P_c) calculation in keratoconus and to develop and validate an algorithm preliminarily to minimize this influence.

Methods:

P_c was calculated theoretically with the classical keratometric approach, the Gaussian equation, and the keratometric approach using a variable keratometric index (n_kadj) dependent on r_1c(P_kadj). Differences in P_IOL calculations (ΔP_IOL) using keratometric and Gaussian P_c values were evaluated. Preliminary clinical validation of a P_IOL algorithm using P_kadj was performed in 13 keratoconus eyes.

Results:

P_IOL underestimation was present if P_c was overestimated, and vice versa. Theoretical P_IOL overestimation up to −5.6 D and −6.2 D using Le Grand and Gullstrand eye models was found for a keratometric index of 1.3375. If n_kadj was used, maximal Δ P_IOL was ±1.1 D, with most of the values ≤±0.6 D. Clinically, P_IOL under- and over-estimations ranged from −1.1 to − 0.4 D. No statistically significant differences were found between P_IOL obtained with P_kadj and Gaussian equation (P > 0.05).

Conclusion:

The use of the keratometric P_c for P_IOL calculations in keratoconus can lead to significant errors that may be minimized using a P_kadj approach.

Impact of corneal cross-linking combined with photorefractive keratectomy on blurring strength

Purpose

The aim of this study was to evaluate the impact of corneal cross-linking combined with photorefractive keratectomy (PRK) on blurring strength.

Methods

A total of 63 patients with keratoconus were recruited for this study, and two study groups were formed according to the therapeutic intervention: corneal collagen cross-linking (CxL) group (33 patients) received corneal cross-linking according to the Dresden protocol, while the rest additionally received topography-guided photorefractive keratectomy (tCxL). The impact of surgical procedure on blurring strength was assessed by power vector analysis. Potential association between blurring strength and vision-specific quality of life was assessed using the National Eye Institute Visual Function Questionnaire (NEI-VFQ) 25 instrument.

Results

Blurring strength presented excellent correlation with NEI-VFQ scores both preoperatively and postoperatively (all P<0.01). Both groups demonstrated nonsignificant changes in best-corrected visual acuity; however, only the tCxL group had significant reduction in blurring strength (13.48+10.86 [preoperative], 4.26+7.99 [postoperative], P=0.042).

Conclusion

Only the combined treatment (tCxL) resulted in significant reduction in blurring strength. Moreover, the excellent correlation of blurring strength with NEI-VFQ scores indicates its reliability as an index of self-reported quality of life in keratoconus, since it seems to address the nonsignificant changes in best-corrected visual acuity following CxL treatments that are conceived as subjective improvement by the patient.